Gas warning devices with a gas sensor must be subjected to function tests at regular intervals. For example, failure of a gas sensor may occur due to blockage of the gas inlet or inactivation of the sensor element. Proper function of a gas sensor is best tested by admitting a target gas, while the entire functional chain from the gas feed to the signal generation is tested.
Commercially available gas sensors have a sensitivity drift with respect to the gas component to be detected. This characteristic of gas sensors cannot be described or predicted by mathematical formulas. It is therefore necessary to calibrate gas sensors within certain time intervals with a target gas of a known concentration. The duration of the interval between calibrations is determined by the requirements imposed on the desired precision of the gas sensor. National specifications require checking of gas sensors at regular intervals.
The effort needed for carrying out a function test and calibration operations is great. For example, testing means, e.g., in the form of pressurized gas containers, preferably containing the target gases, must be kept ready, transported to the gas sensor within the preset duration of use of the gas mixture and finally introduced there into the gas inlet of the gas sensor to be tested through suitable devices, e.g., pumps, valves, calibration adapters and/or flow controllers. To guarantee short test times and sufficient test gas concentrations, dead space volumes and undefined arriving flow conditions must be avoided.
To circumvent these drawbacks, it was already proposed in GB 22 54 696 A1 to accommodate a gas generator together with a gas sensor in a common housing. The common housing is defined here against the gas to be measured by a gas-permeable membrane. Even though the occasional activation of the gas generator makes it possible to test the sensor function with a synthetic gas, the dead space volumes present in the arrangement do compromise the function test. Furthermore, this testing method does not provide any information on the state of the outer, gas-permeable membrane. The path of the gas to the detector electrode thus remains untested.
Test gas is sent through a membrane, which is also connected to the gas generator and the gas sensor, in the gas sensor corresponding to EP 0 744 620 B1. The state of the outer membrane granting access of the gas to the detector electrode can be inferred with difficulty only in this case as well.
In the measuring device corresponding to U.S. Pat. No. 6,635,160 B1, the test gas is injected into a test gas chamber in the interior of the sensor housing, which chamber is arranged downstream of the outer gas inlet. However, the gas inlet from the outside to this chamber and hence also to the detector electrode of the sensor remain untested here as well.
A diagnostic method for gas sensors, in which test gas is pressed through an aperture to a sensor, delivered by adding a propellant or moved to the sensor by thermal expansion, is described in U.S. Pat. No. 4,151,739.
All these embodiments share the drawback that the entire path of the gas sample to the detector electrode is ultimately not tested or means must be provided by mechanically complicated constructions for delivering the test gas to the sensor.